In recent years, for a secondary battery such as a lithium secondary battery, various kinds of materials for positive electrode active material have been studied with the aim of achieving high voltage and high capacity. As such positive electrode active material, for instance, Japanese Patent Provisional Publication Tokkai No. 2008-270201 discloses solid solution material such as Li2MnO3—LiMO2 (M is transition metal whose average oxidation state is 3+).
Regarding the solid solution material disclosed in Japanese Patent Provisional Publication Tokkai No. 2008-270201, depending on its composition etc., there is a case where a hysteresis phenomenon in which an open circuit voltage curve during charge and an open circuit voltage curve during discharge are quite different occurs. Then, when the positive electrode active material showing the occurrence of the hysteresis phenomenon is applied to the secondary battery, due to an influence of the hysteresis phenomenon, the open circuit voltage curve during discharge, of the secondary battery, changes (or differs) depending on an SOC at a start of the discharge. Thus the SOC in a discharge process cannot be properly detected.